Filter and method of manufacturing

ABSTRACT

A filter for use in combination with a cathode ray tube screen to transmit light and absorb other types of energy comprises a translucent plastic substrate onto which is applied a wire mesh impregnated with a thin layer of a bonding agent which constitutes one surface of the filter. In manufacturing the filter, a thin layer of the bonding agent is deposited on the topside of a platen, and the mesh is pressed into the bonding agent to cause it to impregnate the mesh. The translucent plastic substrate is placed on the impregnated mesh, and pressure is applied between the substrate and the platen to form a laminate. Thereafter, the laminate is cured and seperated from the platen, after which it may be placed in a mold and formed into a curved shape.

FIELD OF THE INVENTION

The present invention relates to filters, and more particularly, thepresent invention relates to filters for use in transmitting light whileabsorbing other forms of energy emitted by cathode ray tubes. Thepresent invention also relates to a method of manufacturing suchfilters.

BACKGROUND OF THE INVENTION

In recent years, the widespread use of devices having cathode ray tube(CRTs) has created certain problems. For instance, computer terminals,televisions, and video games are known to have increased the amount ofstray energy emitted into the environment in the vicinity of suchdevices. The art commonly refers to such energy as electromagneticenergy interference (EMI) or radio frequency interference (RFI). Thereis a need, not only to protect people who may be particularly affectedadversely by such interference (such as people having heart pacemakers)but also to protect sensitive electronic equipment from damage caused bysuch interference. Moreover, it is known that such energy can beintercepted and information obtained therefrom by sensitive electronicsurveillance equipment.

While it is not difficult to shield CRT cabinets from both emission of,and penetration by, such forms of energy, it is difficult to providesatisfactory shields, or filters, for the screens of cathode ray tubes,and other information transmitting devices, such as light emittingdiodes (LEDs). This is because of the need for operators to be able toread information from the devices with a minimum of optical distortioncaused by the filter.

U.S. Pat. Nos. 3,305,623; 3,431,348; and 4,247,737 disclose viewingpanels for use with cathode ray tubes to attenuate the transmission ofenergy across the front of the cathode ray tube screen. Each of thesepatented filters is characterized by a loosely knit mesh wire materiallaminated between transparent glass or plastic panels. Most of thefilters of this type are flat, although curved laminated filters arecommercially available.

Viewing panels particularly suited for use with microwave ovens aredisclosed in U.S. Pat. Nos. 4,008,383; 4,010,343; and 4,264,800.

Various shielding devices for use in attenuating electromagnetic energyare disclosed in the following U.S. patents: U.S. Pat. Nos. 2,674,644;2,875,435; 3,030,435; 3,126,440; 3,265,804; and 4,037,009.

Various filters have been proposed for attenuating reflections ofambient light. For example, in U.S. Pat. No. 2,977,412 and U.S. Pat. No.4,253,737, there are disclosed mesh type screens for use in combinationwith cathode ray tubes for this purpose. The former patent suggests theuse of either metallic or synthetic fiber filaments. Neither patent,however, is concerned with the attenuating EMI or RFI energy.

U.S. Pat. No. 4,163,821 discloses an adhesive bonding procedure forproviding mechanical stabilization of metallic glass ribbons in fabricform.

While each of the above noted patented inventions may functionsatisfactorily for its intended purpose, there is a demand for animproved cathode ray tube filter which provides energy attenuationcapabilities, which is free from optical defects, and which is capableof being manufactured efficiently.

OBJECTS OF THE INVENTION

With the foregoing in mind, a primary object of the present invention isto provide a novel filter particularly suited for use in conjunctionwith a cathode ray tube screen to attenuate various types of energypassing through the screen.

It is another object of the present invention to provide an improvedcathode ray tube screen filter which has EMI and RFI energy attenuationcapabilities.

A further object of the present invention is to provide a unique cathoderay tube screen filter having excellent optical properties making itwell suited for those applications where both light transmission and EMIand RFI energy attenuation capabilities are desired.

A still further object of the present invention is to provide animproved method of efficiently manufacturing a novel filter for acathode ray tube screen.

SUMMARY OF THE INVENTION

More specifically, the present invention provides a filter which isadapted to be mounted in front of a cathode ray tube screen to attenuateenergy passing through the screen. The filter comprises a translucentsubstrate onto which a fine mesh of electrically-conductive filaments isbonded by a thin layer of bonding agent. Preferably, the substrate is atransparent plastic panel; the filaments are woven of wire mesh; and thebonding agent is a polyurethane. The wire mesh filaments are exposedalong the peripheral margin of the panel for grounding the panel.

In manufacturing the filter, a layer of the bonding agent is depositedon the topside of a glass platen, and the wire mesh is forced downwardlyinto the layer of bonding agent and is impregnated thereby. The plasticpanel substrate is then laid on the impregnated wire mesh, and pressureis applied between the plastic panel substrate and the platen to ensureintimate contact between the panel and the bonding agent. This assemblyis thereafter cured to cause the impregnated mesh to adhere to theplastic panel substrate and thereby form a laminate. The laminate issubsequently separated from the topside of the platen and may be moldedinto various curved shapes depending on the desired end use. Thepreferred bonding agent includes a photo-initiated curing agent which isactivated when subjected to ultraviolet light during the curing phase ofthe manufacturing process. After curing, the bonding agent is removedfrom the peripheral margin of the filter to expose the wire mesh.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention should become apparent from the following description whentaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view illustrating a typical electronic devicehaving a cabinet with a cathode ray tube screen mounted therein and afilter embodying the present invention mounted in front of the screen;

FIG. 2 is an enlarged fragmentary cross-sectional view taken on line2--2 of FIG. 1;

FIG. 2A is a greatly enlarged fragmentary sectional view of a portion ofthe filter; and

FIG. 3 is a schematic diagram illustrating the process of manufacturingthe filter illustrated in FIG. 2, with FIGS. 3A-3K thereof illustratingindividual steps in the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND METHOD

Referring now to the drawings, FIG. 1 illustrates a conventional cathoderay tube display device 10 with which the filter of the presentinvention finds particular utility. The device 10 comprises a cabinet 11in which is mounted a cathode ray tube having a front face or screen 12adapted to display information. In the illustrated embodiment, thedevice 10 is a computer terminal having a keyboard 13 in front of thescreen 12. While a computer terminal is illustrated, it should beunderstood, that the filter of the present invention may be used with awide variety of electronic devices for which it is desirable to providelight transmission and energy absorption, such as video games,television receivers, light emitting diodes, etc.

As discussed heretofore, the electromagnetic energy emitted from thecathode ray tube screen 12 when the device 10 is in operation can causethe EMI and RFI problems noted heretofore. Moreover, even when thedevice 10 is not operating, it is possible for undesirableelectromagnetic energy to flow in the reverse direction across thescreen 12 and interfere with sensitive electronics housed within thedevice 10. Depending upon the level of the energy emitted or absorbed,and the proximity of sensitive electronic equipment, there is a need toprovide shielding for the screen which both transmits light and absorbsunwanted energy.

In accordance with the present invention, a novel filter 15 has beenprovided to attenuate energy traversing the screen 12. To this end, andas best seen in FIG. 2, the filter 15 is mounted on the front of thecathode ray tube screen 12 in close proximity therewith, such as againstits convex outer surface in the manner illustrated. Preferably, thecabinet 11 is provided with internal resilient supports, such ashighly-conductive resilient gasketing formed of commercially availableconductive elastomeric material, or wire mesh, which engages the filter15 around its entire periphery or bus bars, such as the spring clip 16for engaging the filter 15 at one or more spaced locations along itsperipheral margin as indicated at 15a. Preferably, the clips 16 areelectrically grounded by a suitable conductor 17.

As best seen in FIG. 2A, which is a greatly enlarged fragmentarysectional view of the filter 15, the filter 15 comprises a substrate 20which is preferably formed of a transparent, or at least translucent,plastic. The substrate 20 has a front, or outer, surface 20a and a rearor inner surface 20b. In the illustrated embodiment, the front surface20a is convex and the rear surface 20b is concave and is engaged againstthe front of the cathode ray tube screen 12 and held in place by thespring clip 16.

To absorb the EMI and RFI energy while transmitting light, a mesh 21 ofelectrically-conductive filaments is disposed against the outer surface20a of the panel 20. As best seen in FIG. 2A, a thin layer of a bondingagent 22 impregnates the mesh 21 and covers the surface 20a of the panel20 to a depth sufficient to completely encapsulate the mesh 21. Thelayer 22 provides an outer surface 22a which is parallel with thesurface 20a of the panel 20 and which is substantially coextensivetherewith. The bonding agent 22 adheres the mesh 21 to the panel 20 toprovide a unitary filter assembly 15.

The translucent substrate or panel 20 is preferably fabricated of anacrylic plastic material such as sold by Rohm and Haas Co. ofPhiladelphia, Penn. under the trade designation Plexiglas. Otherplastics which may be used include: polycarbonates, butyrates, vinylsand nylons. While the panel 20 may contain pigments to provide coloringand/or to absorb light of a particular wavelength, the panel 20 ispreferably clear.

The mesh 21 which is superimposed on the panel 20 is preferablyfabricated by weaving very small diameter electrically-conductivefilaments in a conventional manner. Preferably, the filaments are formedof wire drawn from any one of the following metals: beryllium-copper,monel, tin, iron, tungsten, stainless steel and alloys thereof. Thewires may be of a dark color formed by known blackening processes.Desirably, the wires have a diameter in a range of about 1.0 mil. toabout 5 mils. The wires are preferably woven into a fabric of betweenabout 50 mesh to about 300 mesh. Wire mesh of this construction isreadily available from several commercial sources. A woven wire fabricof this construction has been found to be capable of being formed into aconvex shape compatible with the curvatures of conventional cathode raytube screens. While mesh woven of metal wires is preferred, knitted meshmay be utilized, if desired.

The wire mesh fabric 21 is bonded to the plastic panel substrate 20 bythe thin layer of the bonding agent 22 which impregnates the woven wiremesh 21. The thickness of the bonding agent layer 22 is greater than thethickness of the woven wire mesh 20 by a small amount to insure that thesurface 22a of the layer 22 is relatively smooth to the touch.Preferably, the layer 22 has a uniform thickness in a range of about 2.5mils. to about 6 mils., when the woven wire mesh 21 is fabricated fromwire having a diameter in a range of about 1.0 to about 5.0 mils.

The bonding agent 22 must be compatible with the filaments of the mesh21 and the plastic of the substrate 20. Preferably, the bonding agent isa synthetic organic compound such as a polyurethane. A preferredpolyurethane is manufactured by Pittsburgh Plate Glass Co. (PPG), of OakCreek, Wisconsin and sold under the trade designation Raycron 400 UV/EB.It is characterized by a relatively low viscosity, clarity, and lowglossiness. It is an acrylic polymer which has a viscosity of 350-450cps. (Brookfield) and readily forms a film thickness in a 2.0-3.5 mil.range. It is also resistant to abrasion and marring upon curing. While aconventional curing agent may be used with the preferred bonding agentnoted above, a preferred curing agent is of the photo-initiated typesuch as benzophenone or ethyl hexyl acrylate. Such a curing agent isactivated when subjected to ultraviolet light for a brief time period.The bonding agent may include up to about 10%, by weight, of such curingagent, but more preferably in a range of about 2 to about 6%, by weight.

The filter 15 is grounded when installed in the cabinet 11 in the mannerillustrated in FIGS. 1 and 2. To this end, means is provided on thefilter 15 for making electrical contact with the filaments of the meshalong at least a portion of the peripheral margin thereof. In theillustrated embodiment, this is accomplished by providing a recess 22b(FIG. 2A) along at least a part of the periphery of the panel, andpreferably, along the entire periphery. The recess 22b causes the mesh21 to be exposed along the peripheral margin of the filter 15 inwardly asufficient amount (about 1/2 inch) as to engage the electrical contactor bus 16 which is grounded. With this structure, EMI or RFI energypassing in either direction across the filter 15 is absorbed by thewoven wire mesh 21 and conducted to ground via the bus 16 and lead 17.In addition, the mesh 21 functions to scatter and absorb ambient lightimpinging on the filter 15 to provide an anti-glare function,particularly when the filaments of the mesh are blackened.

In the embodiment illustrated, the wire mesh is bonded on the convexside of the substrate 20. While this is preferred, the mesh may bebonded onto the concave side if desired.

The filter 15 is manufactured by the novel process of the presentinvention. The process enables consistently high quality filters to befabricated in an efficient manner without requiring highly skilledtechnicians and sophisticated equipment. The overall process isillustrated schematically in FIG. 3 with individual steps beingillustrated in FIGS. 3A-3K.

Referring now to FIG. 3A, the process begins with there being provided aplaten or work surface 25 larger in size than the area of the filter 15to be fabricated. Preferably, the platen 25 is provided by translucentplate glass having a topside 25'. The topside 25' may be either smoothor may have a mottled or slightly roughened surface, depending upon thedegree of smoothness to be imparted to the outer surface 22a of thebonding agent 22, as will be described. Glass is preferred because itwill not adhere to the bonding agent used to adhere the wire mesh to theplastic substrate.

Referring now to FIG. 3B, the bonding agent is applied as a liquid in athin uniform layer 22 on the topside 25' of the platen 25, as by adispenser 27. The thickness of the layer 22 should be at least 2.5mils., but may be up to about 6 mils., depending on various factors suchas the diameter of the wires in the mesh 21. The area of the layer 22 isat least as great as the area of the filter panel substrate 20. Careshould be taken to insure that the bonding agent is applied evenly tominimize the formation of bubbles, discontinuities, and the like.

After the bonding agent 22 has been applied in a layer on the platen 25,the precut woven wire mesh 21 is laid on the layer 22 in the mannerillustrated in FIG. 3C. The mesh 21 is pressed downwardly into the layer22 to cause the bonding agent 22 to flow into the interstices among thewarp and weft wires and thereby to impregnate the mesh 21. See FIG. 3D.As noted above, the depth, or thickness, of the layer 22 should beselected to ensure that individual wires of the mesh remain below thesurface 22a of the bonding agent layer in the finished filter 15 asshown in FIG. 2A.

Next, the impregnated mesh 21 is bonded to the plastic substrate 20. Asbest seen in FIG. 3E, this is accomplished by aligning the plasticsubstrate 20 with the layer of bonding agent 22 on the platen 25 andlaying the substrate 20 downwardly thereon. Thereafter, downwardpressure is applied between the topside of the plastic substrate 20 andthe platen 25 to ensure intimate contact between the underside of thesubstrate 20 and the impregnated mesh 22. Preferably, pressure isapplied by engaging a roller 29 against the topside of the substrate 20and rolling it back and forth in various directions. The rolling actionpresses the panel substrate 20 firmly downwardly against the impregnatedmesh 21. Alternatively, the pressure may be applied by passing thesubstrate 20 relative to one or more rollers to ensure the desireduniform dispersion.

In order to expel any air or gas bubbles which may be entrained withinthe bonding agent, either during application onto the platen 25 asillustrated in FIG. 3B, or during manipulation of either the mesh 21 orsubstrate 20, it is preferable for the underside of the platen 25 to beilluminated by a source of light, such as the light 30. The light 30 ispreferably of the fluorescent type and of sufficient size as toilluminate the entire underside of the platen 25. The light passingupwardly through the platen 25 also passes through the impregnated mesh21 and the plastic substrate 20 to expose air or gas bubbles and otherdiscontinuities to the technician operating the roller 29 on the topsideof the substrate 20. As a result, the technician is able to adjust therolling action and/or pressure to expel the bubbles and to ensurecontinuous contact between the bonding agent 22 and the entireundersurface of the plastic panel substrate 20.

After all of the bubbles and/or other irregularities in the impregnatedmesh layer 22 have been removed, the bonding agent is cured to hardenits outer surface 22a and to bond the wire mesh 21 to the panel 20. Inthe present invention, the bonding agent includes up to about 10%, butpreferably about 2 to 6%, by weight of the photo-initiated curing agentnoted heretofore. Thus, when subjected to ultraviolet light, such asemitted from the source 30 overlying the plastic substrate 20 in FIG.3G, the curing agent is activated to harden the bonding agent. Ofcourse, should other types of curing agents be utilized, such asthermally-activated curing agents, the light source 30 may be replacedwith a suitable source of heat, such as an infrared lamp.

After curing, the laminated assembly comprising the plastic substrate 20with the bonding agent 22 carrying the mesh 21 is removed from the glassplaten 25. See FIG. 3H. This is accomplished simply by prying the edgesof the laminated assembly 20-21 from the platen 25 and lifting the sameupwardly with respect to the glass platen 25. If desired, the wire mesh21 may be cut larger than needed to cover the panel 20 so that itextends beyond the edges of the panel 20. In such event, the amount ofmesh and bonding agent extending beyond the edge of the panel 20 may betrimmed and discarded.

The panel 15 of the present invention is capable of being molded intovarious curved and convex shapes compatible with the curvatures of thescreens of various cathode ray tubes. To this end, the laminate assembly20-22 is placed in a mold assembly comprising a mating set of a malemold member 31 and a female mold member 32. See FIG. 3I. The moldmembers 31 and 32 may be curved in either a single plane or inorthogonal intersecting planes, depending upon the desired geometry ofthe finished filter 15. While it is preferable for the laminate assembly20-22 to be formed with the impregnated mesh 21 on the convex surface ofthe plastic substrate 20, as shown in FIG. 3I, it may be molded with themesh 21 on the concave side. Molding is accomplished by applying heatand pressure between the mold members 31 and 32 as well known in theart. By way of example, pressure applied in a range of about 0.85 toabout 1.15 psi. and heat applied in a temperature range of about 280° F.to about 300° F. for about 5 to about 10 minutes should yield asatisfactory finished filter 15. After the requisite heat and pressurehas been applied as illustrated in FIG. 3I, the filter 15 is removedfrom the mold and allowed to cool to ambient room temperature. Thefilter 15 has the convex shape illustrated in FIG. 3J.

As noted heretofore, in order to ground the wires of the mesh 21 whenthe filter is installed in a cabinet, the bonding agent 22 is removedalong portions of the outer peripheral margin of the panel 20 to providethe recess 22b and expose the wires of the mesh 21. This may beaccomplished in any one of several ways. One preferred way is simply togrind away the hardened bonding agent 22 by means of an abrasive, orwire, wheel 35, such as indicated schematically in FIG. 3K. Because thewires are metal and more abrasion resistant than the bonding agent 22,this technique ensures exposure of the underlying wires of the mesh 21to enable electrical contact to be made with the contacts 16 mounted inthe cabinet 11 as illustrated in FIG. 2. If desired, the recess 22b maybe formed and the wires exposed by subjecting the margin of the panel 15to a selective solvent which removes only the bonding agent 22. Or, thecentral portion of the panel may be masked and the peripheral marginsand blasted to expose the wires of the mesh 21.

In view of the foregoing, it should be apparent that the presentinvention now provides an improved filter for absorbing EMI and RFIenergy while simultaneously allowing information on a cathode ray tubescreen to be viewed readily. Because the filter is shaped to fit thecurvature of the screen, optical distortions are kept to a minimum.Furthermore, the method of the present invention enables the filter 15to be manufactured economically with a minimum of special equipment andskilled technicians.

While a preferred embodiment of the present invention, and a preferredprocess for making the same, has been described in detail, variousmodifications, alterations and changes may be made without departingfrom the spirit and scope of the present invention as defined in theappended claims.

I claim:
 1. A filter for use in transmitting visible light and absorbingother forms of energy, comprising a translucent substrate, a fine meshof electrically-conductive filaments disposed on said substrate, a thinlayer of bonding agent impregnating said mesh and bonding it to saidsubstrate, said layer of bonding agent constituting a surface of saidfilter having a peripheral margin, and means along at least a portion ofsaid peripheral margin of said filter providing electrical communicationto said filaments.
 2. The filter according to claim 1 wherein said meansproviding said electrical communication includes a recess in saidbonding agent layer exposing outer peripheral ones of said filaments. 3.The filter according to claim 2 wherein said recess extends around theentire peripheral margin of said panel.
 4. The filter according to claim1 wherein said filter has a convex shape with said bonding agent formingthe convex surface thereof.
 5. The filter according to claim 1 whereinsaid filaments have a diameter in a range of about 1.0 to about 5.0mils., and said layer of bonding agent has a uniform thickness in arange of about 2.5 to about 6.0 mils. to provide said filter surface. 6.The filter according to claim 1 wherein said substrate is plastic, saidfilaments are woven of wire mesh, and said bonding agent is a syntheticorganic compound.
 7. The filter according to claim 6 wherein saidplastic includes an acrylic polymer, said wire mesh is of copper, andsaid bonding agent includes a polyurethane.
 8. The filter according toclaim 1 wherein said filaments are of metal wires woven together into afabric of about 50 to about 300 mesh.
 9. The filter according to claim 8wherein said wires include at least one of the followingmetals:beryllium-copper, monel, tin, iron, tungsten, stainless steel andalloys thereof.
 10. The filter according to claim 9 wherein said metalwires have a blackened coating.
 11. The filter according to claim 1wherein said substrate is fabricated of at least one of the followingplastics: acrylics, polycarbonates, butyrates, vinyls, and nylons. 12.The filter according to claim 1 wherein said bonding agent includes apolyurethane having a photo-initiated curing agent.